1. Field of the Invention
The field of art to which this invention relates is an apparatus for polarization conversion. It is more particularly directed to such apparatus which make use of reflective polarizer films and light sources having parabolic mirrors.
2. Discussion of the Prior Art
Intensity in optical projectors is usually limited by source brightness. In addition, projectors that use polarized light, such as many projection displays, suffer further intensity loss if the unused polarization is discarded. Frequently, an effort is made to capture the rejected component and re-inject it into the system after rotating its polarization to match that of the primary component. FIG. 1 illustrates a known arrangement for accomplishing this.
FIG. 1 shows a lamp 102 providing light having both S and P linear polarizations (where S polarization is denoted by xe2x80x9c∘xe2x80x9d and P polarization is denoted by xe2x80x9cxe2x80x9d). The light enters a polarization beam splitter (PBS) cube 104 which transmits light of one of the polarizations, P polarization in this example, and reflects light of the other polarization, S polarization in this example. The reflected S polarized light is again reflected by a mirror 106 and directed towards a xc2xd waveplate 108, where its polarization is converted to P polarization. Thus, the incident light is converted to one polarization and directed to a lens 110 or other optical component.
Two problems with the FIG. 1 approach are added cost and the need to increase optical etendue (NA times field size) when one polarized source image is, by itself, large enough to fill the lens pupil. Increases in etendue add to cost, and the most cost effective step is usually to use a large enough lamp 1 that the image in a single polarization almost fills the pupil aperture chosen; this reduces the benefit from re-injecting the second polarization. Because of arc inhomogeneities, the FIG. 1 arrangement can, in practice, still provide some intensity increase, but the benefit is limited.
A known way to partially circumvent the etendue problem is to recycle the converted polarization through the arc, as shown in FIG. 2. It is thermodynamically impossible to increase the brightness of a black-body source of fixed temperature, but arc discharges are not fully opaque. The FIG. 2 system increases the effective source emissivity by redirecting rays through the arc. (Emissivity equals absorbance, according to Kirchoff""s Law. By tracing rays backwards through the FIG. 2 system, one can see that the recycling elements also increase arc absorbance).
FIG. 2 shows a lamp 102 having a parabolic mirror 102a. The lamp provides both S and P polarized light, both of which pass through a xc2xc waveplate 114 and are directed to a PBS 104. The P polarized light is transmitted 116 and the S polarized light 118 is reflected to a mirror 112. The S polarized light is then reflected back to the PBS 104 and again reflected back towards the parabolic mirror 102a, first passing through the xc2xc waveplate where it is converted to circular polarized light having a right handedness 120. The circularly polarized light having a right handedness 120 is then reflected by the parabolic mirror 102a which converts its handedness to left-handedness 122. This light then reflects of the opposite side of the parabolic mirror 102a which converts its handedness back to right handedness 124. The circular polarized light having a right handedness 124 then passes back through the xc2xc waveplate 114 once again, which converts the light back to linear polarized light but having S polarization 126. The S polarized light 126 is then reflected once again by the PBS 104 towards the mirror 112, and back again towards the lamp 102. In this embodiment, an increase in brightness is not obtained unless there is a phase difference between the S and P components of the reflected light at the parabolic mirror 102a. 
The return mirror 112 in the FIG. 2 system can be slightly tipped so that the two arc images are only partially overlapped; this can improve collected intensity when the system is not fully brightness-limited (due to arc inhomogeneities). However, in practice, the FIG. 2 arrangement is typically reported to have limited efficiency in converting the returned polarization to the desired output state. Also, in most projectors, the PBS 104 in the FIG. 2 system must be added as a new component (though in a few systems, a PBS 104 already present in the optics can also perform the recycling function). A PBS 104 is a fairly expensive optical component.
What is needed is a way to improve the efficiency of recycling and, at the same time, lower its cost.
Therefore, it is an object of the present invention to provide an apparatus for polarization conversion which overcomes the deficiencies of the prior art.
Accordingly, a first embodiment of an apparatus for polarization conversion is provided. The apparatus of the first embodiment comprises a light source for supplying vertically and horizontally linearly polarized light to an optical path and a parabolic mirror disposed in the optical path and proximate to the light source. The parabolic mirror has a mirror coating to induce a phase shift of 90xc2x0 between incident light and reflected light. A polarizer means is disposed in the optical path for reflecting light of one of the linear polarizations and for transmitting the other linear polarization. Lastly, a xc2xc waveplate is disposed in the optical path between the polarizer means and the parabolic mirror. The xc2xc waveplate has quarter wave retardance for converting the reflected linear polarization from the polarizer means to circular polarization before being incident upon the parabolic mirror and for converting the reflected circular polarization from the parabolic mirror to the transmitted polarization which is directed towards, and transmitted by, the polarizer means.
A second embodiment of an apparatus for polarization conversion is also provided. The apparatus of the second embodiment comprises a light source for supplying vertically and horizontally linearly polarized light to an optical path and a parabolic mirror disposed in the optical path and proximate to the light source. The parabolic mirror having a mirror coating to induce a phase shift of 0xc2x0 between incident light and reflected light. A polarizer means is disposed in the optical path for reflecting light of one of the linear polarizations and for transmitting the other linear polarization. Lastly, a xc2xc waveplate is disposed in the optical path between the polarizer means and the parabolic mirror. The xc2xc waveplate has opposing segments each having quarter wave retardance but having axes which are antiparallel to each other, for converting the reflected linear polarization from the polarizer means to circular polarization through one of the segments before being incident upon the parabolic mirror and for converting the reflected circular polarization from the parabolic mirror to the transmitted polarization through the other segment which is directed towards, and transmitted by, the polarizer means.
A third embodiment of an apparatus for polarization conversion is also provided. The apparatus of the third embodiment comprises a light source for supplying vertically and horizontally linearly polarized light to an optical path and a parabolic mirror disposed in the optical path and proximate to the light source. The parabolic mirror has a mirror coating which may induce an arbitrary phase shift between incident light and reflected light. A polarizer means is disposed in the optical path for reflecting light of one of the linear polarizations and for transmitting the other linear polarization. Lastly, a xc2xd waveplate is disposed in the optical path between the polarizer means and the parabolic mirror. The xc2xd waveplate has a plurality of segment pairs. Wherein each individual segment has half wave retardance, opposes the other individual segment of the pair, and has axes which are antiparallel to the opposing individual segment of the pair, for converting the reflected linear polarization from the polarizer means to circular polarization through one of the segments before being incident upon the parabolic mirror and for converting the reflected circular polarization from the parabolic mirror to the transmitted polarization through the opposing segment which is directed towards, and transmitted by, the polarizer means.
A fourth embodiment of an apparatus for polarization conversion is also provided. The apparatus of the fourth embodiment comprises a light source for supplying vertically and horizontally linearly polarized light to an optical path and a parabolic mirror disposed in the optical path and proximate to the light source. The parabolic mirror has a mirror coating which may induce an arbitrary phase shift between incident light and reflected light. A polarizer means is disposed in the optical path for reflecting light of one of the linear polarizations and for transmitting the other linear polarization. A first xc2xc waveplate is disposed in the optical path between the polarizer means and the parabolic mirror for converting the reflected linear polarization to light having a circular polarization. Lastly, a second xc2xc waveplate is disposed in the optical path between the polarizer means and the parabolic mirror, the second xc2xc waveplate having a plurality of segment pairs. Wherein each individual segment has quarter wave retardance, opposes the other individual segment of the pair, and has axes which are antiparallel to the opposing individual segment of the pair, for converting the light having circular polarization from the first xc2xc wave plate to the transmitted linear polarization through one of the segments before being incident upon the parabolic mirror and for converting the reflected transmitted linear polarization from the parabolic mirror to circular polarization through the opposing segment which is directed towards the first xc2xc waveplate, converted to the transmitted polarization thereby, and directed to, and transmitted by, the polarizer means.
In preferred variations of the embodiments of the present invention, the polarizer means is a reflective polarizer film, such as DBEF manufactured by the 3M Corporation.